The present invention relates to a method of crystallizing an amorphous silicon film, and more particularly, to a crystallization method using sequential lateral solidification (SLS).
Polycrystalline silicon (p-Si) and amorphous silicon (a-Si) are often used as active layer materials for thin film transistors (TFTs) in liquid crystal display (LCD) devices. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, amorphous silicon (a-Si) is commonly used in switching devices of liquid crystal displays (LCDs). Unfortunately, amorphous silicon (a-Si) TFTs have relatively slow display response times that limit their suitability for large area LCD.
In contrast, polycrystalline silicon TFTs provide much faster display response times. Thus, polycrystalline silicon (p-Si) is well suited for use in large LCD devices, such as laptop computers and wall-mounted television sets. Such applications often require TFTs having a field effect mobility greater than 30 cm2/Vs together with low leakage current.
A polycrystalline silicon film is composed of crystal grains having grain boundaries. The larger the grains and the more regular the grains boundaries the better the field effect mobility. Thus, a silicon crystallization method that produces large grains, ideally a single crystal, would be useful.
One method of crystallizing amorphous silicon into polycrystalline silicon is sequential lateral solidification (SLS). SLS crystallization uses the fact that silicon grains tend to grow laterally from the interfaces between liquid and solid silicon such that the resulting grain boundaries are perpendicular to the interfaces. With SLS, amorphous silicon is crystallized using a laser beam having a magnitude and a relative motion that melts amorphous silicon such that the melted silicon forms laterally grown silicon grains upon re-crystallization.
FIG. 1A is a schematic configuration of a conventional sequential lateral solidification (SLS) apparatus, while FIG. 1B shows a plan view of a conventional mask 38 that is used in the apparatus of FIG. 1A. In FIG. 1A, the SLS apparatus 32 includes a laser generator 36, a mask 38, a condenser lens 40, and an objective lens 42. The laser generator 36 generates and emits a laser beam 34. The intensity of the laser beam 34 is adjusted by an attenuator (not shown) in the path of the laser beam 34. The laser beam 34 is then condensed by the condenser lens 40 and is then directed onto the mask 38.
Specifically referencing FIG. 1B, The mask 38 includes a plurality of light transmitting areas A through which the laser beam 34 passes, and light absorptive areas B that absorb the laser beam 34. The width of each light transmitting area A effectively defines the grain size of the crystallized silicon produced by a first laser irradiation. Furthermore, the distance between each light transmitting area A defines the size of the lateral grains growth of amorphous silicon crystallized by the SLS method. Referring now to FIG. 1A, the objective lens 42 is arranged below the mask and reduces the shape of the laser beam that passes through the mask 38.
Still referring to FIG. 1A, an X-Y stage 46 is arranged adjacent to the objective lens 42. The X-Y stage 46, which is movable in two orthogonal axial directions, includes an x-axial direction drive unit for driving the x-axis stage and a y-axial direction drive unit for driving the y-axis stage. A substrate 44 is placed on the X-Y stage 46 so as to receive light from the objective lens 42. Although not shown in FIG. 1A, it should be understood that an amorphous silicon film is on the substrate 44, thereby defining a sample substrate.
To use the conventional SLS apparatus, the laser generator 36 and the mask 38 are typically fixed in a predetermined position while the X-Y stage 46 moves the amorphous silicon film on the sample substrate 44 in the x-axial and/or y-axial direction. Alternatively, the X-Y stage 46 may be fixed while the mask 38 moves to crystallize the amorphous silicon film on the sample substrate 44.
FIGS. 2A-2C are cross sectional views showing one block of an amorphous silicon film being crystallized using a conventional SLS method. In the illustrated crystallization, it should be understood that the mask (see FIGS. 1A and 1B) has three light transmitting areas.
Before performing SLS crystallization, a buffer layer 12 is typically formed on a substrate 10, as shown in FIG. 2A, and then, an amorphous silicon film 14 is deposited on the buffer layer 12. Thereafter, the amorphous silicon will be crystallized as described above using the SLS apparatus. The buffer layer 12 is usually formed of an inorganic material, such as silicon nitride (SiNX) or silicon oxide (SiO2). The buffer layer 12 functions as preventing a diffusion of impurities from the substrate 10 to the amorphous silicon film 14 during a later performed laser irradiation.
The amorphous silicon film 14 is usually deposited on the buffer layer 12 using chemical vapor deposition (CVD). Unfortunately, that method produces amorphous silicon with a lot of hydrogen. To reduce the hydrogen content, the amorphous silicon film 14 is typically thermal-treated, which causes de-hydrogenation, which results in a smoother surface on the crystalline silicon film. If the de-hydrogenation is not performed, the surface of the crystalline silicon film is rough and the electrical characteristics of the crystalline silicon film are degraded.
Now in FIG. 2B, a mask 38 having light transmitting areas A and light absorptive areas B is located above the amorphous silicon film 14, and then laser irradiation is performed through the mask 38. Therefore, the amorphous silicon film 14 is partially melted so that it includes melt regions C and non-melt regions D. The melt regions C correspond to the light transmitting areas A of the mask, and the non-melt regions D correspond to the light absorptive areas B.
Next in FIG. 2C, melted silicon of the melt region C is rapidly crystallized, so that grains 60a and 60b laterally grow from the interface between the melt region C and the non-melt region D. Namely, lateral grain growth of the grains 60a and 60b proceeds from the un-melted regions D to the fully melted regions C. The grains 60a laterally growing on left and the grains 60b laterally growing on right collide with each other in a center of the melt region C. Therefore, each melt region C has a first grain region E and a second grain region F after the first laser irradiation.
Lateral growth stops when: (1) grains grown from interfaces collide near the center of the melted region C; or (2) polycrystalline silicon particles are formed in the center of the melted regions C as the melted silicon region solidifies sufficiently to generate solidification nuclei.
If the width of the light transmitting areas A (see FIG. 1B) is equal or less than the maximum lateral growth of the grain, the grains 60a and 60b collide with each other in the center of the region C and then stop growing. Alternatively, if the width of the light transmitting areas A (see FIG. 1B) is twice as large as the maximum lateral growth of the grain, the width of the melted silicon regions C is also twice as large as the maximum lateral growth length of the grain. Therefore, the lateral grain growth stops when the polycrystalline silicon particles are formed in the center of the region C. Such polycrystalline silicon particles act as solidification nuclei in a subsequent crystallization step.
FIGS. 2D-2F are plan views showing the lateral grain growth using sequential lateral solidification (SLS) after the first irradiation.
Especially, FIG. 2D is a plan view of FIG. 2C. As described above, each of the melted regions C is divided into the first and second grain regions E and F after the first laser irradiation. Furthermore, grain boundaries of the first and second regions E and F tend to form perpendicular to the interface between the first and second regions E and F.
FIG. 2E illustrates crystallizing the silicon film of FIG. 2D using a second laser beam irradiation. After the first laser beam irradiation, the X-Y stage or the mask 38 moves in a direction along the lateral grain growth of the grains 60a or 60b (in FIG. 2C), i.e., in the X direction, by a distance that is no more than the maximum length of the lateral grain growth. Then, a second laser beam irradiation is conducted.
In order to grow the grains, the mask 38 moves (relative to the amorphous silicon) to a position where the light transmitting area(s) A exposes a portion of the first grain region E, the border between the first and second grain regions E and F, the second grain region F, and a portion of the amorphous silicon region. Namely, the light transmitting areas A should overlap the borders between the first and second grain regions E and F and between the second grain region and the amorphous silicon region. That is because a lot of defects, such as dislocation and lattice defects, occur around the borders. Namely, such defects are generated at the edges of the laser beam.
After moving the X-Y stage or the mask 38, the second laser beam irradiation is conducted to grow grains formed by the first laser beam irradiation, thereby resulting in a third grain regions G as shown in FIG. 2E. As a result of the second laser beam irradiation, the grains 60a on the left grow until they collide with grains growing right to left in the third grain region G.
Accordingly, by repeating the above-mentioned foregoing steps of melting and crystallizing, one block of the amorphous silicon film is crystallized to form large grains 60c as shown in FIG. 2F. The above-mentioned crystallization processes conducted within one block are repeated block by block across the amorphous silicon film. Therefore, the large size amorphous silicon film is converted into a crystalline silicon film. While generally successful, the conventional SLS method described above has disadvantages.
After completing the crystalline silicon film, an optical unevenness is produced in portions where the laser beams are overlapped between the laser beam irradiations. That is because the thermal energy of the laser beam affects the underlying buffer layer (reference number 12 of FIG. 2A-2C). Therefore, a new method of crystallizing amorphous silicon using sequential lateral solidification (SLS) is required for improving manufacturing efficiency.